Mishnayos Makkos Perek 1 Mishnah 4

Witnesses are not rendered conspiring witnesses until the witnesses who come to render them conspiring impeach the witnesses themselves and not merely their testimony. How so? A set of witnesses said: We testify with regard to a man called so-and-so that he killed a person, and they attested to the precise time and place that the murder took place. Then, a second set of witnesses came to court and said to them: How can you testify about this event? This person who was killed, or this person who killed, was with us, i.e., with the second set of witnesses, on that day in such and such place, which is not the location identified by the first set of witnesses. In that case, although the second set of witnesses contradicted the testimony of the first set, these first witnesses are not rendered conspiring witnesses. But if the second set of witnesses came to court and said to them: How can you testify about that event? You were with us on that day in such and such place. In this case, these first witnesses are rendered conspiring witnesses, and are executed on the basis of their, i.e., the second set’s, testimony.

אֵין הָעֵדִים נַעֲשִׂים זוֹמְמִין עַד שֶׁיָּזוֹמוּ אֶת עַצְמָן. כֵּיצַד, אָמְרוּ מְעִידִין אָנוּ בְאִישׁ פְּלוֹנִי שֶׁהָרַג אֶת הַנֶּפֶשׁ, אָמְרוּ לָהֶן הֵיאַךְ אַתֶּם מְעִידִין, שֶׁהֲרֵי נֶהֱרָג זֶה אוֹ הַהוֹרֵג הָיָה עִמָּנוּ אוֹתוֹ הַיּוֹם בְּמָקוֹם פְּלוֹנִי, אֵין אֵלּוּ זוֹמְמִין. אֲבָל אָמְרוּ לָהֶם הֵיאַךְ אַתֶּם מְעִידִין, שֶׁהֲרֵי אַתֶּם הֱיִיתֶם עִמָּנוּ אוֹתוֹ הַיּוֹם בְּמָקוֹם פְּלוֹנִי, הֲרֵי אֵלּוּ זוֹמְמִין, וְנֶהֱרָגִין עַל פִּיהֶם: